1. Field of the Invention
The present invention relates to a method of manufacturing an inexpensive and good-quality hollow steering shaft of a steering apparatus off a vehicle and such a steering shaft.
2. Related Background Art
A conventional steering apparatus of a vehicle has a structure as shown in FIG. 10, for example. A steering shaft 1 is supported only rotatably in a steering column 2 supported on a vehicle body. A steering wheel (not shown) is fixed to the upper end of the steering shaft 1. The movement of the steering wheel is transmitted to steering gears (not shown) through a universal joint 3 and a transmission shaft 4.
The steering apparatus comprising the steering shaft 1 and the steering column 2 of the conventional type has a so-called collapsible structure which shrinks in its longitudinal direction when it receives a shock so that it protects the driver at the time of collision. As shown in FIG. 10, the steering shaft 1, for example, comprises a hollow cylindrical lower shaft 5 and a solid upper shaft 6 connected thereto. Each of the fitting portions of both shafts 5 and 6 has an elliptical cross section so as to prevent relative rotation therebetween. Synthetic resin members 9 are filled and solidified in annular grooves 7 Formed in the outer peripheral surface of the lower portion of the upper shaft 6 and through holes 8 formed in the upper portion of the lower shaft 5. The synthetic resin members 9 prevent axial displacement between the shafts 5 and 6 in the normal operation of a vehicle. Upon collision, however, they are broken and allow the displacement between the shafts 5 and 6 to shorten the length of the steering shaft 1.
Recently, there have been used more hollow upper shafts 6a of steering shafts 1, as shown in FIGS. 11 and 12, in order to make the steering shafts light. Each hollow upper shaft 6a is manufactured by drawing a cylindrical blank tube having a circular cross section. On the upper end portion of the upper shaft 6a are Formed a spline portion 10 and a male screw 11 which engages a nut For holding a steering wheel mounted on the spline portion 10.
The lower half of the upper shaft 6a forms a substantially elliptical fitting portion 14 comprising a pair of arcuate portions 12 and a pair of flat portions 13 arranged circumferentially and alternately. The fitting portion 14 is inserted in a fitting portion 15 formed on the upper half of the lower shaft 5 (FIG. 10) so that only axial movement between the fitting portions 14 and 15 is allowed. The annular grooves 7 are formed in the outer peripheral surface of the fitting portion 14.
Conventionally, the fitting portion 14 has been formed on the lower half of the upper shaft 6a as shown in FIGS. 13A to 13E. First, a cylindrical blank tube 16 to be formed into an upper shaft 6a is disposed so as to face a drawing die 18 fixed in a holding case 17, as shown in FIG. 13A. As shown in detail in FIGS. 14 to 16, the die 18 has a land portion 19 and a tapered portion 20 having cross sectional areas which become smaller as they approach the land portion 19. The inner face of the land portion 19 takes a substantially elliptical shape which is complementary to the outer peripheral surface of the fitting portion 14. The shape of the cross section of the inner face of the tapered portion 20 gradually changes from a circle to a substantial ellipse toward the land portion 19, and an intermediate portion of the tapered portion 20 takes a shape as shown in FIG. 16.
After an end of the blank tube 16 has been disposed so as to face the die 18 at its large diameter side, a mandrel 21 is inserted into the die 18 from the opposite side to the blank tube 16 and then the front end portion 22 of the mandrel 21 is inserted into the blank tube 16, as shown in FIG. 13B. The shape of the front end portion 22 is similar to the shape of the inner face of the land portion 19, i.e., substantially elliptical.
After the front end portion 22 of the mandrel 21 has been inserted into the blank tube 16, the blank tube 16 is pushed into the die 18. In synchronism with the pushing-in of the blank tube 16, the mandrel 21 is pulled out of the die 18. During this operation, the blank tube 16 is tightly held between the inner face of the land portion 19 and the outer peripheral surface of the mandrel 21 and plastically deformed so as to become substantially elliptical in cross section.
After an elliptically cross-sectioned portion having an ample length has been formed by fully inserting the front end portion of the blank tube 16 into the die 18, the mandrel 21 is pulled out of the blank tube 16, as shown in FIG. 13D, and then the blank tube 16 is also pulled out of the die 18. The blank tube 16 having the required portion drawn so as to be substantially elliptical in cross section is transported to a station where annular grooves 7 are formed.
However, the conventional method of manufacturing a hollow steering shaft in which the required portion of a blank tube 16 is drawn to form a substantially elliptical cross section is subject to the Following problems to be solved.
(1) In order to manufacture a hollow steering shaft having a high quality, the dimensional accuracies, particularly the thickness accuracy, must be controlled very strictly. PA1 (2) An apparatus for inserting the mandrel and pulling the same out is required. This makes the structure of a shaping apparatus complicated and increases the apparatus cost. PA1 (3) Since the cross sections of the blank tube 16 are changed by holding the blank tube 16 between the land portion 19 of the die 18 and the mandrel 21 under a large force and by squeezing the blank tube 16 to reduce its thickness, the shaping load is very large. Thus, during shaping operation, a high surface pressure is applied to the land portion 19, and the land portion 19 and the outer peripheral surface of the blank tube 16 rub with each other, resulting in quick wear of the land portion 19. In consequence, the die 18 must be changed very frequently, resulting in a high manufacturing cost. PA1 (4) Since the shape of the cross sections is changed by reducing the thickness off the blank tube 16 as described above, the blank tube 16 is elongated during the drawing operation, and it is difficult to control the elongation accurately. Thus, a post-process is required for cutting the blank tubes 16 to the same length. This also contributes to a high manufacturing cost.
When the thickness is too large, use of the mandrel 21 as shown in FIG. 13C remarkably increases a shaping load required for pushing the blank tube 16 into the die 18. As a result, the buckling or the like adverse phenomenon occurs to the blank tube 16 due to the shaping load, hindering smooth shaping operation.
When the thickness is too small, on the other hand, an intermediate part of each flat portion 13 of the elliptically cross-sectioned part of the blank tube 16 is bent inward, and/or the thickness of the connecting portions 23 between the Flat portions 13 and the arcuate portions 12 becomes smaller than the required thickness.
When a hollow steering shaft is manufactured, the mandrel 21 which causes the above-mentioned problems might be omitted. If, however, the mandrel 21 is omitted, intermediate parts of the flat portions of a blank tube are likely to be bent inward and/or the connecting portions between the flat portions and the arcuate portions off the blank tube are likely to have insufficient thickness, as is in the case where the thickness is too small.
A hollow steering shaft might be manufactured by the use of a rotary swaging machine. In this case, a mandrel having an elliptical cross section is inserted into a blank tube having a circular cross section, and then the cross section of the blank tube is formed into a substantially elliptical shape by hitting the outer peripheral surface of the blank tube so that the inner diameter of the blank tube reduces. However, this manufacturing method requires a long operation time and a high apparatus cost, and noise is generated during the operation. Thus, this method is not practical.